I can share you some of my observations, because I am working on a project to get 8+ GPUs into same machine.
Now, I have been confirmed by NVIDIA tech people, that this is possible and it is not limited by drivers or OS:
https://devtalk.nvidia.com/default/topic/1004967/cuda-programming-and-performance/max-number-of-cuda-devices/I have also contacted several manufacturers that are creating pcie expansion backplanes, such as:
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http://amfeltec.com/-
http://www.onestopsystems.com/-
http://cyclone.com/index.phpOut of curiosity, I also ordered that cheap 30 USD 1x to 3x PCIe expansion that is being sold everywhere around. Tested on several various motherboards from low end to high end, it does NOT work!
Then I ordered backplane from amfeltec which has a PLX chip on. It works, no issues at all. But of course, these toys are not for mining. A very simple backplane that has only 4 slots (1x to 4x) costs around 500 USD. And here comes the trick; all ports on backplane works fine, you can put 4 cards on it and it will work, but the total number of GPUs in system matter. And the motherboard I used for testing this backplane supports only 6 GPUs. No matter where I insert 7th card, the system would not boot. This indicate that the real issue is in BIOS and it is not a hardware problem.
An interesting expansion backplane is this:
http://www.acalbfi.com/nl/Embedded-computing/PCI-PCIe/Bus-Expansion/p/PCIe-x16-Gen-2-Expansion/00000008SX It is the size of ATX motherboard. But keep in mind that this toy costs ~1500 USD.
I also sent query to the companies listed above asking about motherboard compatibility. I was specifically asking whether an Asus X99-E-10G WS (a 500 USD worth motherboard) would work. Their response was:
... I would recommend higher-end desktop motherboards or server-class machines. Low end desktops tend to have BIOS packages that take shortcuts. Your Asus mobo may be perfectly fine. We have successfully used Intel, SuperMicro, Asus, Dell, HP, and Gigabyte with our 16 slot backplane. ...
Bottom line is this; forget about these cheap expansions methods. Even if they work, your motherboard probably will not support it. Cheap motherboards
tend to take shortcuts, which would explain everything. If you really want a solution, you will first need pro expensive motherboard. After that, you will need even more expensive expansion backplane. The only pcie switching chips that actually work are PLX and these chips are EXPENSIVE. Ever wondered why motherboards that support 4way CF/SLI are so expensive? Because they pack one or two PLX chips that cost 100 USD a piece. And forget about cheap chinese shit (just look at that 1x to 3x PCIe expansion board, it has some cheap chinese chip on top of it, no wonder it doesn't work). It is also highly unlikely that some chinese company would come out with their own designed motherboard; we all know that all they do is just copy and copy from west world and are incapable of designing stuff on their own.
several months ago i've tried the 1-to-3 multiplier, bought on ebay.
it works fine, mining with 3 nitro rx470.
the mainboard i use was biostar a68n-5000.
i think thats what the card do, multiply a mobo with only 1 or 2 slots.
with other mainboard such as gigabyte 5 slot it can work only 6 gpus.
rgds,
